Linear beam tube having a beam collector cooled by radiation through an infrared window



June 3, 1969 HEN ET AL 3,448,325

LINEAR BEAM TUBE HAVING A BEAM COLLECTOR COOLED BY RADIATION THROUGH ANINFRARED WINDOW Filed Sept. 6, 1966 Fi i? :2 o

Q T n a INVENTORS ERSING L. LIEN ILLAM J. EDIGH BY 04 RNEY United StatesPatent 3,448,325 LINEAR BEAM TUBE HAVING A BEAM COLLEC- TOR COOLEI) BYRADIATION THROUGH AN INFRARED WINDOW Erling L. Lien, Los Altos, andWilliam J. Leidigh, Belmont, Calif., assignors to Varian Associates,Palo Alto, Calif., a corporation of California Filed Sept. 6, 1966, Ser.No. 577,440 Int. Cl. H01j 25/34 US. Cl. 315-3.5 9 Claims The inventiondescribed herein was made in the performance of work under a NASAcontract and is subject to the provisions of section 305 of the NationalAeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42U.S.C. 2457) The present invention relates in general to linear beamtubes and, more particularly, to such tubes employing a beam collectorelectrode which is cooled by radiation through a gas tight infrared wavepermeable window. Such tubes are especially useful for, but not limitedin use to, portable microwave velocity modulation power tubes as may beemployed, for example, in satellite communication systems or fortransmitting information from deep space probes.

Heretofore, radiation cooling of refractory beam collector electrodesthrough a glass vacuum envelope has been employed in power grid tubes.In such tubes, the beam collector electrode surrounds the grid andcathode electrodes and could not operate above a relatively lowtemperature, as for example, 1000 C. since it acts like an oven andwould otherwise overheat the grid and cathode emitter in use. Theradition efliciency of such a radiator is limited by the operatingtemperature of the radiating element.

Radiation cooling of the beam collector electrodes in linear beamvelocity modulation tubes has been employed. However, in these priortubes the radiating beam collector element formed the vacuum envelope ofthe tube and was typically made of a good thermal conductor such ascopper. The problem with this type of collector is that it must operateat a relatively low temperature as of 500 C. to prevent melting thereofand to maintain sufiicient strength to hold out the atmosphericpressure. As a consequence, the radiation cooling efiiciency isrelatively low and the collector becomes excessively large and heavy formany portable applications.

In the present invention, the beam of a linear beam tube is collected ina refractory metal collector electrode operating at a relatively hightemperature, as of greater than 1000 C. The collector electrode iscooled by radiation through an infrared Wave permeable vacuum tightwindow portion of the tubes vacuum envelope. In a preferred embodimentof the present invention, the radiating collector electrode operatesbetween 1300 C. and 1500" C. and is positioned adjacent a thermallyshielded infrared reflector which reflects back directed infrared energythrough the infrared window and away from the tube. In addition, asuppressor electrode, operating at a negative potential relative to thebeam collector, is positioned adjacent the beam entrance thereof todrain positive ions from the collector to prevent ion focusing insidethe collector. When the collector is operated as a depressed collectorfor increased overall radio frequency conversion efiiciency, thenegative suppressor electrode also prevents thermionic electronsgenerated inside the collector from flowing back to the R.F. circuit ordown the beam path toward the cathode electrode. Collectors of thepresent invention have achieved 75% radiation efficiency at 300 wattsC.W. input with 20,000 hours expected operating life.

The principal object of the present invention is the ice provision of animproved linear beam tube having a radiation cooled beam collector.

One feature of the present invention is the provision of a heatradiating refractory metallic beam collector at the terminal end of alinear beam tube with the collector radiating its infrared energy awayfrom the tube through a separate infrared wave permeable vacuum tightwindow, whereby the beam collector may be operated at temperatures inexcess of 1000 C. to provide substantial radiating eificiency forcooling the collector of the tube.

Another feature of the present invention is the same as the precedingfeature including a thermally shielded infrared reflector positioned atthe end of the tube between the collector and the main body of the tubefor reflecting infrared energy emitted from the collector out the end ofthe tube through the infrared Window, whereby the cooling efficiency ofthe collector is enhanced.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the radiating collector electrode hasits radiating surface roughened and/or coated with a black coating toincrease the radiating efllciency of the collector.

Another feature of the present invention is the same as any one or moreof the preceding features including the provision of a suppressorelectrode positioned near the beam entrance opening to the beamcollector and operated at a negative potential relative to the beamcollector for draining positive ions from the collector and forpreventing thermionic emission from the collector from flowing back downthe beam path to the R.F. circuit and toward the cathode electrode.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

FIG. 1 is a longitudinal cross sectional schematic diagram of a linearbeam tube incorporating features of the present invention.

FIG. 2 is an enlarged sectional view of the structure of FIG. 1delineated by line 22, and

FIG. 3 is a sectional view of the structure of FIG. 2 taken along theline 33 in the direction of the arrows.

Referring now to FIG. 1 there is shown a linear beam tube 1incorporating the radiation cooled beam collector structure 2 of thepresent invention. More particularly, the tube 1 is an electrostaticallyfocused extended interaction klystron. The klystron tube 1 has a cathodeelectrode 3 at one end operating in conjunction with a focus electrode 4and anode electrode 5 to form and project a beam of electrons over anelongated linear beam path 6 to a hollow beam collector electrode 7 atthe terminal end of the tube 1.

An electromagnetic interaction circuit 8 is disposed intermediate thecathode 3 and beam collector 7 for cummulative interaction with the beamto produce amplification of microwave signal wave energy applied to theinput end of the circuit 8 via an input R.F. coupler 9. The amplifiedsignal wave energy is extracted from the interaction circuit 8 viaoutput R.F. coupler 11 and fed to a suitable utilization device or load,such as an antenna, not shown.

The interaction circuit 8 comprises a plurality of extended interactionhelix resonators 12 successively disposed along the beam path 6 andseparated by microwave field free drift regions 13. The helix resonators12 each comprise a length of metallic tape conductor, as of molybdenum,wound in the shape of a helix 14 and connected at opposite ends to theside walls of a conductive metallic chamber 15, as of molybdenum. Theinput resonator 12 has its resonant helix coupled to the end of thecenter conductor of the coaxial R.F. input coupler 9. The end walls ofthe cavities 15 are centrally apertured for passage of the beamtherethrough.

In helical resonators 12 the molybdenum tape helix 14 is supported byone beryllia and two quartz rods, not shown. The beryllia rod serves toconduct the heat from the helix 14 to the molybdenum shell 15. The helixassembly 14 is simply clamped in place by the resonator shell 15. Thehelical resonator 12 operates at its lowest resonant frequency and iselectrically half a wavelength long. Due to end effects, in particularthe presence of the beam hole, the variation of the axial component ofthe electric field is approximately sinusoidal. The helix 14 is,therefore, as far as beam interaction is concerned, effectively a fullwavelength long. In order to minimize the variation of the beam couplingcoeflicient over the operating voltages, the helix length is adjusted toa peak-to-peak separation in the electric field corresponding to anelectronic phase shift fl =2.5 radians at a beam voltage of 3 kv. At thesame voltage the normalized inner radius of the helix 14 is 1.13radians. The value of the interaction impedance (R /Q) is 230 ohms andthe square of the beam coupling coefficient M is 0.58.

To obtain 30 MHz for the tube 1 bandwidth at 2.3 gigahertz, the bunchercavities 12 are stagger-tuned. Tuning of the helical resonator 12 isaccomplished by perturbing the radial component of the electrical field.Using a radially oriented conductive cylindrical tuning plunger, notshown, located half way along the length of the resonator 12 where theradial field is strongest, up to 100 MHz. tuning is easily realized withonly a 5 percent decrease in R Q. The buncher cavities 12 serve tovelocity modulate the beam with the signal energy and to successivelyincrease the current density modulation of the beam bunches insuccessive drift regions 13.

The output circuit comprises a double gap 7r mode cavity resonator 16having conically shaped resonator end walls. The distance between thepenultimate resonator 12 and the center of the output resonator 16 isselected for optimum bunching at the output interaction gaps. Thecollector electrode entrance 17 is located as close to the outputinteraction gaps as possible because of the rapid spread of the beamunder R.F. condition. The resulting interaction impedance (R /Q) of theoutput resonator 16 is 285 ohms.

The output resonator 16 is coupled via loop 18 to the coaxial outputcoupler 11. Tuning of the output resonator 16 is by means of aninductive plungernot shown. The 21r-mode resonant frequency for cavity16 is 3550 MHz. The output loop 18 satisfactorily loads both the 1r and211'- modes of resonance so that the 21r-mode loaded Q is about 220. Thedegree of 21r-mode unloading required to start oscillation isapproximately a factor two.

The output resonator shell and walls are fabricated from copper-cladmolybdenum. The drift tube 19 of the output cavity 16 and its singlesupport .arm 21 are made from molybdenum. Copper plating is used toreduce the RF. conduction losses, resulting in a circuit efficiency of96.7 percent.

The tube 1 is electrostatically focused by means of a plurality of ringelectrodes 22 operated at cathode poten tial and successively axiallyspaced apart along the beam path 6. Potential supply leads 23 for thering electrodes pass through insulator assemblies 24 in the walls of avacuum envelope 25 of the tube 1. The vacuum envelope structure 25 isevacuated to a low pressure as of torr, and encloses the cathode 3,interaction circuit 8 and beam collector electrode 7. The outputterminal end of the vacuum envelope is sealed by a disk-shaped infraredwave permeable window member 26 as of sappbire or quartz approximately 3inches in diameter and 0.125" thick.

An outwardly flared, generally parobolic shaped, infrared reflectorstructure 27 is disposed between the beam collector electrode 7 and theinteraction circuit 8 for refleeting infrared wave energy emitted fromthe collector electrode 7 through the infrared window 26 and away fromthe tube 1 for cooling thereof. The beam collector electrode 7 andreflector 27, in a preferred embodiment, operate at a potential negativewith respect to the interaction circuit 8 to form a depressed collector,thereby enhancing the overall R.F. efficiency of the tube 1.

A cathode potwer supply 28 supplies the cathode-toanode operatingpotential as of 3 -kv. relative to the grounded anode 5 and interactioncircuit 8. An anode-tocathode cylindrical insulator 29 forms a portionof the vacuum envelope structure 25 and holds off the anode-tocathodepotential. A separate beam collector power supply 31 supplies thecathode to beam collector operating potential, as of +2 kv. Thus, thebeam collector 7 and reflector 27 are operated at a negative potentialof 1 kv. relative to the grounded interaction circuit 8 and anodeportion of the envelope structure 25, thereby obtaining the increasedR.F. efficiency attendant the depressed collector mode of operation.

In operation, the input signal velocity modulates the beam. The beam isfurther velocity modulated and current density bunched by the staggertuned buncher cavities 12. The bunched beam excites the output cavity16. The amplified output signal is extracted from the output cavity 16.The spent beam is collected in the collector electrode 7, therebyheating the collector to a temperature falling within the range of 1300"C. to 1500 C. At these temepratures the collector is an efiicientthermal radiator, radiating the major part of its energy in the form ofinfrared ray-s out the end of the tube through the sapphire window 26.

In a typical example of an s band tube 1, the tube :provided RF. poweroutput of watts with 35 MHz bandwidth, with 42 db. of gain at36%-overall conversion efliciency. For this case, the radiation cooledcollector must dissipate 200 watts at 75% radiation efficiency. Theother 25% of the energy is removed from the collector region by thermalconduction and radiation back down the beam path 6.

Referring now to FIGS. 2 and 3 the beam collector feature of the presentinvention will .be described in greater detail. The collector electrode7 is formed by a hollow generally egg-shaped member having an invertedfrustoconical beam entrance portion 35 closed by a domeshaped potrion 36which receives the beam incident thereon. This shape produces even beaminterception over its interior beam collecting surfaces as the beamexpands due to space charge repulsion. The electrode 7 is preferablymade of chemically vapor deposited tungsten having a wall thickness ofabout 0.010". The vapor deposition leaves the external surface rough dueto the tungsten crystallite orientation which is normal to the surface.This rough exterior surface increases the radiation efliciency of thecollector 7. In addition, the radiation efiiciency may be furtherincreased by coating the exterior surface of the electrode 7 with ablack coating of zirconium carbide.

The collector electrode 7 is carried from a surrounding support cylinder3-7, as of stainless steel with a wall thickness of 0.200", via a pairof orthogonally crossed rods 38. The rods 38, as of 0.040" diametertungsten rhenium alloy, are anchored at their ends in oval Moneldiaphragrns 39 as of 0.005" wall thickness brazed at their margins intooval cutout portions of the support cylinder 37. The lower rod 38' isdownwardly bowed while the upper rod 38 is upwardly bowed. The rods 38are connected to the side Walls of the collector electrode by tantalumeyelets 41, spot welded to the collector 7 and rods 38 and through whichthe rods 38 pass. The center portions of the rods 38, within thecollector electrode, are flattened to a thickness of 0.029" to present athin beam interception profile to the beam. The oppositely directedbowing of the rods 38 serves to retain the collector electrode 7 in thesame position in spite of thermally produced changes in the dimensionsof the rods 38, electrode 7 and surrounding support structure 37. For a300 watt dissipation collector '5 electrode 7, the electrode 7 is about2 inches in length and 1.25" in diameter at its largest point. The rods38 are about 2.75" in length.

The generally parabolic shaped infrared reflector 27 is made of 0.005 to0.010" thick sheet of polished refractory metal having a high infraredreflectivity such as tantalum, molybdeum, or tungsten. The reflector 27is outwardly flared and positioned facing the infrared permeable window26 to reflect the infrared radiation, received by the reflector from thecollector 7, through the window and away from the tube 1. The reflector7 has an axial length of about 2.25" and a diameter at its largest pointof 2.75". The reflector 27 is supported at its lip 42 from the end ofthe cylindrical support 37, as by spot Welding.

A pair of thermal shields 43, substantially identical to the reflector27, are closely and coaxially positioned surrounding the outside of thereflector 27 and spaced therefrom and from each other 'by dimples formedin the mutually opposed surfaces thereof. The reflector 27 and thermalshields 43 are centrally apertured to a diameter of about 0.350" topermit passage of the beam theretlhrough. The thermal shields inhibitback streaming of heat from the reflector 27 to the interaction circuit8. The reflector 27, heat shields 43, and collector electrode 7 aresupported from the output cavity 16 via the intermediary of acylindrical ceramic insulator 44 which, in addition, permits theseparate depressed beam collector potential to be supplied to thoseelements relative to the anode potential applied to the interactioncircuit 8.

The infrared window 26 is brazed at its periphery to a cylindricalcopper nickel alloy window frame member 45. The frame 45 is outwardlyflanged at 46 for sealing as by Welding to a similarly flanged portion47 of the cylindrical vacuum envelope 25, as of copper nickel alloy toform a vacuum tight joint therebetween. The window 26 is spaced byapproximately 0.125" from the closest point of the collector electrode7.

The beam collector and radiation structure, with dimensions asaforecited, dissipated 300 watts of beam power with 75% radiationefliciency. The structure will dissipate about 1 kw. beam power with thesame efficiency with about a 30 to 50% increase in the diameter of thebeam collector and radiation structure. In this case the beam collectorwould also be coated with a black coating. With further increases indimensions, a structure of this design is capable of dissipating up tokw. of beam power with the same radiation efliciency.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A linear beam tube apparatus including, means forming a cathodeelectrode for forming and projecting a beam of electrons over anelongated linear beam path, means forming a refractory metallic beamcollector at the terminal end of the linear beam path for collecting anddissipating the energy of the beam by infrared radiation, means disposedalong the beam path intermediate said cathode electrode and said beamcollector for electromagnetic interaction with the beam to produce anoutput microwave signal, means for extracting the output signal forapplication to a utilization device, means forming an evacuated envelopestructure for enclosing said cathode electrode, said interaction means,and said. beam collector electrode, means forming an infrared wavepermeable window structure disposed at the terminal end of the tubeapparatus adjacent said beam collector electrode and forming a portionof said envelope structure for passing infrared wave energy emitted fromsaid beam collector through said envelope structure and away from thetube apparatus for cooling thereof, whereby the tube apparatus is cooledby radiation.

2. The apparatus of claim 1 including, means forming an outwardly flaredinfrared reflector disposed facing said window means adjacent saidcollector electrode and within said envelope structure between saidcollector electrode and said electromagnetic interaction means forreflecting infrared energy emitted from said collector electrode throughsaid infrared window means, thereby inhibiting infrared radiation fromtraveling back toward said interaction means and thus producing enhancedcooling of the tube apparatus.

3. The apparatus of claim 2 including means forming a thermal shieldstructure disposed between said reflector means and said interactionmeans, for additionally shielding said interaction means from infraredradiation traveling back toward said interaction means.

4. The apparatus of claim 1 including means forming a suppressorelectrode disposed between said beam collector electrode and saidinteraction means, and operated at a negative potential relative to theoperating potential of said beam collector electrode for suppressingthermionic electron emission from said beam collector electrode fromtraveling back to the RF. circuit and along the beam path in thedirection toward said cathode electrode and for draining positive ionsfrom the interior of said beam collecting electrode to prevent ionfocusing of the beam therein.

5. The apparatus of claim 1 wherein an outer surface portion of saidbeam collector electrode is provided with a rough exterior surface toenhance infrared radiation therefrom.

6. The apparatus of claim 5 wherein the rough exterior surface portionof said beam collector includes a black coating.

7. The apparatus of claim 1 wherein said infrared window is formed of amaterial selected from the class of sapphire and quartz.

8. The apparatus of claim 3 wherein said infrared reflector means andsaid thermal shield means comprises a plurality of coaxially disposedflared members disposed in spaced apart relationship over substantialmutually opposed surfaces areas thereof.

9. The apparatus of claim 1 including, means forming an electricalinsulator disposed intermediate said interaction means and said beamcollector means for operating said beam collector means at a potentialnegative with respect to said interaction means, whereby the beamcollector may be operated as a depressed collector for enhancing radiofrequency efliciency of the tube apparatus.

References Cited UNITED STATES PATENTS 2,860,277 11/1958 Iversen 313-45X 2,957,103 10/1960 Birdsall 313-45 X 3,026,435 3/ 1962 McPherson 313-45X JAMES W. LAWRENCE, Primary Examiner. C. R. CAMPBELL, AssistantExaminer.

US. Cl. X.R.

1. A LINEAR BEAM TUBE APPARATUS INCLUDING, MEANS FORMING A CATHODEELECTRODE FOR FORMING AND PROJECTING A BEAM OF ELECTRONS OVER ANELONGATED LINEAR BEAM PATH, MEANS FORMING A REFRACTORY METALLIC BEAMCOLLECTOR AT THE TERMINAL END OF THE LINEAR PATH FOR COLLECTING ANDDISSIPATING THE ENERGY OF THE BEAM BY INFRARED RADIATION, MEANS DISPOSEDALONG THE BEAM PATH INTERMEDIATE, SAID CATHODE ELECTRODE WITH THE BEAMCOLLECTOR FOR ELECTROMAGNETIC INTERACTION WITH THE BEAM TO PRODUCE ANOUTPUT MICROWAVE SIGNAL, MEANS FOR EXTRACTING THE OUTPUT SIGNAL FORAPPLICATION TO A UTILIZATION DEVICE, MEANS FORMING AN EVACUATED ENVELOPESTRUCTURE FOR ENCLOSING SAID CATHODE ELECTRODE, SAID INTERACTION MEANS,AND SAID BEAM COLLECTOR ELECTRODE, MEANS FORMING AN INFRARED WAVEPERMEABLE WINDOW STRUCTURE DISPOSED AT THE TERMINAL END OF THE TUBEAPPARATUS ADJACENT SAID BEAM COLLECTOR ELECTRODE AND FORMING A PORTIONOF SAID ENVELOPE STRUCTURE FOR PASSING INFRARED WAVE ENERGY EMITTED FROMSAID BEAM COLLECTOR THROUGH SAID ENVELOPE STRUCTURE AND AWAY FROM THETUBE APPARATUS FOR COOLING THEREOF, WHEREBY THE TUBE APPARATUS IS COOLEDBY RADIATION.